This project is concerned with the relation of structure of dihydrofolate reductase (dihydrofolate plus NADPH yields tetra-hydrofolate plus NADP from Methotrexate-resistant bacterial and L1210 leukemia cells. Using several techniques of protein chemistry, the problems to be investigated include: (a) conformational studies of protein structure upon substrate, coenzyme and inhibitor binding using circular dichroic (cd), magnetic CD and fluorometric techniques; (b) chemical modification of specific amino acid residues on the protein and measurement of the effect of these modifications upon the binding of substrates and inhibitors and upon catalytic activity; (c) amino acid sequence studies of the entire enzyme and peptides containing functional amino acid residues; (d) examination of inhibition properties of selected quinazolines and other compounds toward bacterial and mammalian dihydrofolate reductases; (e) determination of transport characteristics of selected quinazoline derivatives in L1210 lymphoma cells; (f) examination of selected quinozolines as inhibitors of dihydrofolate reductase and transport properties in certain human drug-sensitive and drug-resistant leukemic leukocytes; (g) design and evaluation of active site-directed irreversible inhibitors of dihydrofolate reductase.